In recent years, the silver halide photographic light-sensitive materials are more and more strictly required to have high sensitivity, excellent graininess, superior gradation, high sharpness, good storability or suitability for rapid processing of development rate. In particular, a demand for still higher sensitivity while suppressing the fog low and keeping good storability is strong.
The silver halide emulsion is usually subjected to chemical sensitization using various chemical substances so as to obtain desired sensitivity and gradation.
Specific examples of the chemical sensitization include chalcogen sensitization such as sulfur sensitization, selenium sensitization and tellurium sensitization, noble metal sensitization using a noble metal such as gold, and reduction sensitization using a reducing agent. These sensitization methods are used individually or in combination.
With respect to the time for performing the reduction sensitization, the time of forming silver halide grains has been studied as described, for example, in JP-A-48-87825 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-50-3619 and European Unexamined Patent Publication Nos. 348934A, 369491A, 371388A, 396424A, 404142A and 435355A.
In the reduction sensitization of these publications, reducing compounds described in T. H. James, The Theory of the Photographic Process, Chap. 5, Macmillan (1977), P. Grafikides, Chimie et Physique Photographique, 5th ed., Paul Montel (1987), and Research Disclosure, vol. 307, No. 307105 have been predominantly used. Specific examples of the reducing compound include aminoiminomethanesulfinic acid (also called thiourea dioxide), borane compounds (e.g., dimethylaminoborane), hydrazine compounds (e.g., hydrazine, p-tolylhydrazine), polyamine compounds (e.g., diethyltriamine, tiethylenetetramine), stannous chloride, silane compounds, reductones (e.g., ascorbic acid), sulfite, formaldehyde and hydrogen gas.
Further, it has also been studied to perform reduction sensitization after forming the silver halide grains and in this case, in addition to the use of the above-described reduction sensitizer (i.e., reducing compounds), an attempt has been made to reduction-sensitize the surface of a silver halide grain using a reduction sensitizer described in JP-A-8-272024 and U.S. Pat. No. 5,500,333.
The sensitization center (i.e., nuclei of sensitization) of yielding high sensitivity by the reduction sensitization is considered to be a small silver nucleus which is very readily changed by aging particularly under the conditions of allowing the presence of moisture and air. This is the reason for poor storability of the silver halide emulsion subjected to reduction sensitization and also for the tendency of fog to worsen. The small silver nucleus is produced inside a silver halide grain but the silver nucleus cannot be completely fixed to that position and along the deposition of silver halide thereon, a fairly large part of the silver core comes out to the grain surface and may readily cause fog or give rises to poor storability.
Accordingly, in order to control the silver nucleus which readily causes fog, an oxidizing agent such as thiosulfinic acid or disulfide compound is used together as described in some patent publications described above, however, if the oxidizing agent is used too much, there arise problems that the sensitivity is reduced or the latent image is liable to regress.
Under these circumstances, it is keenly demanded to improve storability and fog while maintaining the high sensitivity of the silver halide emulsion subjected to reduction sensitization.